Definition
Cel shading is a special graphics technique that makes 3D graphics look like hand-drawn 2D comics or animation. Instead of smooth gradients, it shows light and shadow as a few distinct flat bands of color, and adds thick, crisp outlines to create its signature look.
Painting in Cartoon Colors Instead of Smooth Realism
Put a 3D clay figure under a lamp, and the color flows smoothly from the lit areas to the dark ones. 3D computer graphics are normally calculated to imitate this natural shading (the change between light and shadow). Cel shading boldly throws away that smooth gradient.
It gives the computer clear-cut rules, such as: "If the light is above 70%, use the brightest color. Between 30% and 70%, use a middle color. Below that, use the dark shadow color." This wipes out the complex curves of the 3D form. Light and shadow become two or three crisp bands of flat color, and the image instantly looks like a flat cartoon drawing.
The name "cel shading" comes from how it resembles the old way of making 2D animation: tracing lines in ink on clear film (a cel, short for celluloid) and filling in paint one flat tone at a time.
The Black Outline That Makes a Character Pop
Cutting down the color steps alone doesn't finish the cartoon look. The most important thing that gives comics their sharp, clear feel is the bold, crisp outline around a character's body and clothing.
A 3D model is made of countless polygon surfaces, so it doesn't naturally have any outlines. So the graphics program calculates the angle between the camera's line of sight and the object's surface in real time. It finds the edges where the surface curves away from your view and paints black lines over them on purpose. Another trick is to inflate the model slightly and paint only its back faces black, which makes a border.
To be more precise, modern cel shading also uses post-processing. It analyzes the screen pixel by pixel, spots places where neighboring colors or depth change sharply, and draws border lines there.
A Look That Doesn't Go Out of Style
Realistic graphics that look just like real people can start to look dated as soon as the technology moves forward a little. Games and animation made with cel shading, on the other hand, are remembered for their own unique art style, so they're often said to age more gracefully over the years.
At first it only imitated 2D comics, but it has evolved to the point where it can finely control the shine of hair in sunlight and even the shadow of a collar fluttering in the wind. That's why games like The Legend of Zelda and anime-style action games use it so actively.
It has another advantage too. Unlike realistic graphics, it doesn't need to calculate every reflection and refraction of light with extreme precision. So it can deliver a consistent art style while keeping the screen's computing load efficient.
Common misconceptions
Cel-shaded graphics are just lazy 2D, made that way because the technology wasn't good enough.
Artists carefully build the full 3D model and skeleton in 3D space, then change the way light and shadow are calculated (the shader) so it looks like a 2D cartoon. It's a deliberate expressive technique.
Where you meet it
3D games like The Legend of Zelda: Breath of the Wild, which feel like you're adventuring inside a cartoon world
Japanese animated feature films that build characters with 3D modeling but recreate the feel of smooth 2D drawn animation
In one sentence
A rendering technique that simplifies a 3D model's smooth shading into flat bands of color and adds outlines, so it looks like a 2D cartoon.